1. Field of the Invention
This invention relates to fuel cells and more particularly to means for removing waste heat from fuel cells.
2. Description of the Prior Art
A fuel cell power section comprises a plurality of individual cells electrically connected in series. The cells convert reactants, such as a hydrogen containing fuel and air or other oxidants, into DC electrical power in a manner well known in the art. The cells are housed between separator plates which provide flow passages to bring the reactants to the cell. The electrochemical reaction produces, as a by-product, waste heat which must be removed in a controlled manner to maintain the cells at the desired operating temperature. For efficient operation it is desirable to maintain the cells at a uniform temperature and at a maximum level consistent with material compatibility characteristics.
A well-known method for removing waste heat from a fuel cell power section is to use heat exchange surfaces parallel to the plane of the cells. These heat exchange surfaces often take the form of passageways through the separator plates forming conduits which carry a coolant fluid. The intimate contact between the coolant fluid and the separator plates provides high heat transfer capabilities between the cells and the cooling medium thereby minimizing the temperature gradient therebetween. Depending upon the power density and thermal properties of the fuel cell power section, one cooler could be used between every cell or one cooler could be used to remove heat from several cells, the latter being more typical.
Since the coolant system is an integral part of the power section, it is exposed to the electrical potentials of the cells. In large power sections this can be hundreds or even thousands of volts. It is important, therefore, that there be no appreciable flow of electrical current (i.e., shunt currents) between the cells and ground through the cooling loop. These shunt currents could cause serious corrosion of power section components and/or piping and could result in potentially large parasitic power losses. For this reason fuel cell cooling systems have traditionally used dielectric fluids as the coolant since they cannot conduct electric current. Conduits feeding the dielectric coolant to the coolant passageways were electrically insulated from the stack such as by surrounding the connecting end with dielectric material. Although this eliminates the problems associated with shunt currents in the cooling loop, there are associated disadvantages. For example, dielectric fluids, such as fluorocarbon or silicon based oils, which are capable of operating at fuel cell temperatures are expensive. The low specific heat of dielectric coolants require high mass flow rates through the cells, with a resultant loss of power due to the energy consumed by the coolant pumps. The higher flow rates require larger flow passage sizes leading to an increase in the size and cost of the power section and its connecting plumbing.
In addition to the foregoing, there are other disadvantages of using dielectric coolants. The amount of heat transferred to the dielectric coolant is a function of the difference between its temperature at the inlet to the cell and its temperature at the exit of the cell. If the cell temperature is not allowed to go above a certain maximum level, a majority of the cell area will necessarily operate at a temperature lower than this maximum temperature resulting in a temperature skew across the cell. This reduces the cell output and overall efficiency. Also, cells are highly sensitive to dielectric fluids. Even trace amounts of a dielectric coolant leaking into the cells can seriously degrade or even totally ruin cell performance. This potential problem is in addition to the fact that the dielectric coolants are flammable and have toxic products of reaction. The foregoing problems are complicated by the fact that dielectric coolants have low surface tension properties which makes them extremely difficult to seal.